Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a sheet.
Description of the Related Art
An image forming apparatus has a stacking unit for stacking sheets. As the stacking unit, a feed cassette and a manual bypass tray provided within the image forming apparatus are used. If the number of sheets that exceeds the number expected in design is stacked in these stacking units, a feeding failure may occur. Japanese Patent Laid-Open No. 05-278896 proposes an image forming apparatus that detects over stacking by measuring the height of a sheet bundle stacked in a cassette, using a sensor.
However, if the height of a bundle of envelope type sheets is detected using a sensor, over stacking is erroneously detected in some cases. Since air is likely to accumulate in an envelope type sheet, the envelope type sheet can be easily pressed down. A feed cassette is provided with a locking claw for regulating the number of stacked sheets, but a large number of envelope type sheets are forcibly over-stacked by pressing down the large number of envelope type sheets in some cases. Thus, since the height of the sheet bundle is visually low, over stacking of the sheets cannot be accurately detected even with the sensor for detecting the height of the sheet bundle.